TJ-II Stellarator Research Articles

Overview of the TJ-II stellarator research programme towards model validation in fusion plasmas

TJ-II stellarator results on modelling and validation of plasma flow asymmetries due to on-surface potential variations, plasma fuelling physics, Alfvén eigenmodes (AEs) control and stability, the interplay between turbulence and neoclassical (NC) mechanisms and liquid metals are reported. Regarding the validation of the neoclassically predicted potential asymmetries, its impact on the radial electric field along the flux surface has been successfully validated against Doppler reflectometry measurements. Research on the physics and modelling of plasma core fuelling with pellets and tracer encapsulated solid pellet injection has shown that, although post-injection particle radial redistributions can be understood qualitatively from NC mechanisms, turbulence and fluctuations are strongly affected during the ablation process. Advanced analysis tools based on transfer entropy have shown that radial electric fields do not only affect the radial turbulence correlation length but are also capable of reducing the propagation of turbulence from the edge into the scrape-off layer. Direct experimental observation of long range correlated structures show that zonal flow structures are ubiquitous in the whole plasma cross-section in the TJ-II stellarator. Alfvénic activity control strategies using ECRH and ECCD as well as the relation between zonal structures and AEs are reported. Finally, the behaviour of liquid metals exposed to hot and cold plasmas in a capillary porous system container was investigated.

The impact of radial electric fields and plasma rotation on intermittence in TJ-II

This work explores the impact of an imposed radial electric field on the intermittence parameter in magnetically confined plasmas. The intermittence is sensitive to both the magnetic configuration (dominant helical modes or low order rational surfaces) and to poloidal flows or radial electric fields. This behaviour was verified both in numerical turbulence calculations using a resistive magnetohydrodynamic model, and using Langmuir probe data obtained in experiments at the TJ-II stellarator. It is shown that the intermittence parameter can be used to detect when the local plasma rotation velocity, with respect to the laboratory frame of reference, is minimum.

Plasma diagnostic systems and methods used on the stellarator TJ-II

The TJ-II is a heliac-type stellarator device with major radius of 1.5 m and averaged minor radius ⩽0.22 m that has been operated at CIEMAT, Madrid since 1998. Its full magnetic field is created by a system of poloidal, central, toroidal and vertical field coils, thus it possesses a fully 3-dimensional plasma structure and a bean-shaped plasma cross-section. Although this results in a complicated vacuum-vessel layout, it has excellent port access for diagnostics (96 portholes). During its initial operational phase, it was equipped with a limited set of essential diagnostics. Since then, a broad variety and large number of both passive and active diagnostics have been installed. The former include Hα monitors, light spectrometers, an electron cyclotron emission radiometer, X-ray filter monitors, neutral particle analysers, magnetic diagnostics, as well as cameras, among others, while the latter include various laser, atomic and ion beam based diagnostics, microwave probe beams, Langmuir probes plus impurity injection techniques. In this paper, after describing the TJ-II stellarator, its heating and fuelling systems, the diagnostic systems employed are outlined and discussed briefly here. Finally, results obtained with selected systems are highlighted.

Observation of low frequency electrostatic coherent oscillation in the proximity of the density limit in the TJ-II stellarator

Turbulence self-regulation mechanisms are widely believed to be partially governed by the non-linear interplay between turbulence and large-scale plasma flows. In this paper, we employ floating potential data recorded via Langmuir probes to study the evolution of long-range correlated structures in plasmas near the operational density limit of the TJ-II stellarator. The results point towards the presence and amplification of low frequency, coherent, global fluctuations with similar properties to those of the zonal flows in the vicinity of this limit.

A lithium laser-ablation based time-of-flight (LILA-TOF) diagnostic for measuring plasma edge ion temperature and toroidal plasma rotation

A new method for studying the thermalization and transport of injected impurities at the edge of hot plasma, (considering the last closed magnetic surface, the free path is between 1 and 2 cm approx.) under no perturbative conditions, is presented. In the proposed technique, a Nd:YAG laser is used to ablate Li from the lithiated wall of the stellarator TJ-II. While the laser pulse allows for the analysis of the released species through laser induced breakdown spectroscopy (LIBS), its laser pulse also provides a time reference for the time-of-flight (TOF) measurements of the Li+ ions performed. This is done by positioning light detection systems sensitive to an intense Li II spectral line at different toroidal locations away from such a source. TOF times of tens to hundreds of microseconds are recorded. Then, by de-convolving the shape of the recorded light pulse, the velocity distribution of the lithium-ion during its thermalization with the background plasma can be extracted. From this velocity distribution, the ion temperature of the background ions and the toroidal rotation at the plasma periphery can be deduced. In contrast with conventional Doppler spectroscopy, this technique uses filter-scope detectors rather than high-spectral-resolution spectrometers thus a tradeoff between spectral and time or space resolution is required. Finally, preliminary results of the application of this novel technique are shown to validate this proof of principle.

Topology of 2D turbulent structures based on intermittence in the TJ-II stellarator

This work estimates the degree of turbulent intermittence of the plasma potential measured by a heavy ion beam probe in the core plasma region of the TJ-II stellarator. It is shown that the intermittence varies in a significant way with the plasma state (ion or electron root). In addition, radial minima of the intermittence are found to be associated with the location of topological structures of the flow associated with some important low-order rational surfaces. The local pressure gradient was also estimated, and a clear correlation was found between the steepening of the pressure gradient and the deepening of the minima of the intermittence, suggesting that the minima are associated with pressure gradient driven modes. By estimating the rotation velocity of the plasma from the measured plasma potential, it was possible to make a rough reconstruction of the two-dimensional radial–poloidal map of intermittence, thus clarifying the topological structure of the intermittence. The experimental results were put into context by comparing with simulations performed using a resistive magneto-hydrodynamic turbulence model.

Causality, intermittence, and crossphase evolution during confinement transitions in the TJ-II stellarator

In this work, we study spontaneous electron to ion root transitions in TJ-II using Langmuir probes. By scanning the probe position on a shot to shot basis, we reconstruct a spatiotemporal map of the evolution of important turbulent quantities in the plasma edge region. We pay particular attention to the evolution of the cross phase between transport-relevant variables, showing the spatiotemporal evolution of this quantity for the first time, revealing the outward propagation of the changes associated with the transition. We also compute the intermittence parameter, which allows us to conclude that the turbulence, although its amplitude increases, condenses in a reduced number of dominant modes and becomes less bursty. The causal relationship between variables is studied using the transfer entropy, clarifying the interactions between the main variables and offering a rather complete picture of the complex evolution of the plasma across the confinement transition.

Evolution of Heavy Ion Beam Probing from the Origins to Study of Symmetric Structures in Fusion Plasmas

The overview discusses development of the unique fusion plasma diagnostics—Heavy Ion Beam Probing (HIBP) in application to toroidal magnetic plasma devices. The basis of the HIBP measurements of the plasma electric potential and processing of experimental data are considered. Diagnostic systems for probing plasma in tokamaks TM-4, TJ-1, TUMAN-3M and T-10, stellarators WEGA, TJ-II and Uragan-2M are presented. Promising results of the HIBP projects for various existing modern machines, such as TCV, TCABR, MAST, COMPASS, GLOBUS-M2, T-15 MD and W7-X and the international fusion tokamak reactor ITER are given. Results from two machines with similar size and plasma parameters, but with different types of the magnetic con-figuration: axisymmetric tokamak T-10 and helically symmetric stellarator TJ-II are compared. The results of studies of stationary potential profiles and oscillations in the form of quasimonochromatic and broadband fluctuations, turbulent particle flux, fluctuations of density and poloidal magnetic field are presented. The properties of symmetric structures—zonal flows and geodesic acoustic modes of plasma oscillations as well as Alfvén Eigenmodes excited by fast particles from neutral beam injection heating are described. General trends in the behavior of electric potential and turbulence in magnetized fusion plasmas are revealed.

Automatic identification of MHD modes in magnetic fluctuation spectrograms using deep learning techniques

The control and mitigation of magnetohydrodynamic (MHD) oscillation modes is an issue in fusion science because these modes can contribute to the outward particle/energy flux and can drive the device away from ignition conditions. It is of general interest to extract the mode information from large experimental databases in a fast and reliable way. We present a software tool based on deep learning that can identify these oscillation modes taking Mirnov coil spectrograms as input data. It uses convolutional neural networks that we trained with manually-annotated spectrograms from the TJ-II stellarator database. We have tested several detector architectures, resulting in a detector area under the curve score of 0.6 on the test set. Finally, it is applied to find MHD modes in our spectrograms to show how this new software tool can be used to mine large databases.

Nuclear Fusion Pattern Recognition by Ensemble Learning

Nuclear fusion is the process by which two or more atomic nuclei join together to form a single heavier nucleus. This is usually accompanied by the release of large quantities of energy. This energy could be cheaper, cleaner, and safer than other technology currently in use. Experiments in nuclear fusion generate a large number of signals that are stored in huge databases. It is impossible to do a complete analysis of this data manually, and it is essential to automate this process. That is why machine learning models have been used to this end in previous years. In the literature, several popular algorithms can be found to carry out the automatic classification of signals. Among these, ensemble methods provide a good balance between success rate and internal information about models. Specifically, AdaBoost algorithm will allow obtaining an explicit set of rules that explains the class for each input data, adding interpretability to the models. In this paper, an innovative approach to perform an online classification, that is, to identify the discharge before it actually ends, using interpretable models is presented. In order to evaluate and reveal the benefits of rule-based models, an illustrative example has been implemented to perform an online classification of five different signals of the TJ-II stellarator fusion device located in Madrid, Spain.

The interpretation of magnetic activity associated with pellet injections into plasmas created in the stellarator TJ-II

Pellet injection into magnetic confinement plasma is widely employed for plasma fuelling and impurity transport studies. While post-injection effects include changes in density and temperature, several short-lived effects have been identified during and immediately after pellet ablation, e.g. pre-cooling of the plasma core or a strong reduction in fluctuations. Here, transitory effects observed in magnetic signals during and immediately after the injection of fuel and tracer encapsulated solid pellets into microwave and neutral beam heated plasma in the stellarator TJ-II are reported. One effect is the excitation of a clear, short-lived, magnetic instability during the ablation phase of a cryogenic pellet. Another effect is a brief reduction in broadband magnetic fluctuations immediately after an injection while a third effect is frequency jumps of already excited Alfvén eigenmodes (AE) when neutral beam heating is employed. The interpretation of these effects is facilitated by signals from 2 recently installed toroidal arrays of magnetic coils that follow the helicity of this device along one of its quadrants. It is concluded that interactions between outward drifting pellet particles and a low-order rational surface lead to the magnetic instability, that pellet-induced plasma cooling induces a transitory reduction in broadband magnetic fluctuations and that plasma mass density changes dominate jumps in AE frequencies.

Stability analysis of TJ-II stellarator NBI driven Alfvén eigenmodes in ECRH and ECCD experiments

In this paper, we analyze the impact of electron cyclotron resonance heating and electron cyclotron current drive on the Alfvénic instabilities driven by neutral beam injection observed in the TJ-II stellarator. An MHD stability analysis of driven Alfvén eigenmodes compatible with the experimental plasma parameters is carried out in order to compare with the data provided by magnetic coils, radiation monitors, and heavy ion beam probes. To this end, the vacuum magnetic configuration modified by the different levels of plasma current, the thermal plasma parameters and the fast ion pressure profiles generated by the co-injected neutral beam, are entered in the FAR3d gyro-fluid code in order to follow the linear evolution of the destabilized plasma equilibrium. Linear growth rates and radial location of the dominant predicted modes coincident in frequency with the observed fluctuations are presented. Despite the uncertainties related to the estimation of the rotational transform profile, the code predictions agree within reasonable accuracy with the experimental results.

Spatial characterization of edge zonal flows in the TJ-II stellarator: the roles of plasma heating and isotope mass

In this article, we report the latest results for the impacts of the isotope mass and the plasma heating scenario on the spatiotemporal characteristics of the low-frequency zonal flow (ZF)-like electrostatic oscillation found in the plasma edge of the TJ-II stellarator. The radial profile of the toroidal cross-coherence in low-frequency floating potential fluctuations (1 < f < 25 kHz), known as long-range correlations (LRCs), was explored along the plasma boundary region by a dual Langmuir probe system. The study was performed using electron cyclotron resonance heating (ECRH)- and neutral beam injection (NBI)-heated plasmas with two different isotope concentrations: pure hydrogen plasmas and plasmas dominated by deuterium (). The highest toroidal coherence levels are detected for floating potential oscillations at less than 10 kHz in hydrogen and deuterium plasmas, in both the ECRH and NBI heating schemes. The radial extent of the LRC profile is clearly larger in the ECRH than in the NBI-heated plasma scenarios. While the maximum amplitude of the LRC does not show a significant dependence on isotope mass, the relation between the LRCs’ radial sizes in plasmas and different isotope masses increases with the isotope mass in NBI-heated scenarios. These results provide the first experimental observation of ZFs with a larger radial size in deuterium than in hydrogen plasmas.

High power characterisation of the ECRH transmission lines and power deposition calculations in the TJ-II stellarator

Abstract In the TJ-II stellarator, the last mirror of both ECRH transmission lines is an in-vessel steerable phase correcting mirror that distorts the power distribution of the reflected beam whenever the mirror orientation angles deviate from the design position. Beam distortions cannot be measured due to space restrictions when accessing the inner part of the device, particularly if we have in mind the wide range of orientation angles needed to carry out ECRH and ECCD experiments, for which such experimental characterisation should be performed. Instead, a set of full wave electromagnetic simulations allows us to estimate the power distribution of the beam coupled to the plasma and how this distribution changes when the in-vessel mirror is steered. High power characterisation of the transmission lines is first accomplished and documented in order to determine the beam power distribution before entering the vessel. For the perpendicular injection case, the additional contribution to the beam distortion produced by its passage through the structure supporting the window is calculated. Leaving aside the power losses due to beam trimming (7% and 10% for the first and second line respectively), the added distortion produced by the window housing is of minor importance compared to the effect of the internal mirror. The impact of beam distortion on the width of the power deposition profiles for several launching directions is addressed using the ray/beam tracing code TRUBA. Results show that the beam waist in the plasma appears smaller and ahead of the position predicted by quasi optical theory, leading to beam widening in the magnetic axis where the resonant layer is located. Despite the evident distortion, power deposition profiles calculated with beam tracing, refined with a Gabor expansion of the wave field distribution in case of strongly oblique incidence, do not differ much from the predictions of the standard ray tracing calculation.

Design and Testing of Advanced Liquid Metal Targets for DEMO Divertor: The OLMAT Project

In a future fusion reactor like DEMOnstration reactor (DEMO) one of the main concerns is the handling of the power exhaust from the plasma, especially at the divertor. The expected power loads cannot easily be handled by traditional armor solutions based on solid materials like tungsten, especially when the effect of intense neutron bombardment is also considered. Interest in armor concepts based on liquid metals has been subsequently on the rise, as they prove to be more resilient against high, fast power loads and neutron bombardment. However, engineering solutions for those concepts are very complex, and need to be tested. For this purpose, Optimization of Liquid Metal Advanced Targets project (OLMAT) has been envisaged. The project will use the Neutral Beam Injection of the TJ-II stellarator to irradiate liquid metal targets with power densities (neutrals plus occasionally ions) relevant to DEMO steady state operation, in the range of 20 MW/m2. OLMAT design will allow a series of experiments that other divertor simulator devices cannot easily perform: in-situ measurements of hydrogen retention, redeposition, vapor shielding, material fatigue, dust and precipitates effects, etc. Moreover, a high-power fiber laser will be used to simulate Edge Localized Modes in a small area, or to simulate the strike point power deposition profile.

Boron synergies in lithium film performance

Fusion experiments use lithium and boron getters to capture impurities and control the density of fuel species in the plasma. In the TJ-II Stellarator, the addition of a boron-carbon substrate was found to extend the lifetime of the very reactive lithium wall conditioning. However, the synergies between the lithium layer and the underlying boron-carbon, or the effect of a glow discharge cleaning, on oxygen and deuterium gettering are not fully understood. Laboratory experiments were therefore initiated to gain deeper insight on the getter properties of B(C)/Li walls. The coatings were exposed to oxygen gas and plasmas while the capture and release of O was followed by mass spectrometry. Without the boron substrate, the maximum oxygen uptake is half the number of lithium atoms in the film for both oxygen gas and oxygen plasma. Conversely, the behaviour of B(C)/Li walls depends on the nature of the oxidation process: molecular oxygen (O:Li ≈ 0.15) or plasma (O:Li ≈ 0.50). The oxygen plasma gettering is thus preserved against molecular oxidation, e.g. an overnight exposure to the residual gas. While oxygen is known to promote hydrogen retention in lithium, the oxygen content of a B(C)/Li coating shows little effect on deuterium retention. Unlike simple lithium, the oxygen and deuterium gettering of B(C)/Li recovers after a helium glow discharge treatment. These advantageous features clearly point to a change in chemistry, with complex interactions between film constituents and the oxygen-rich environment of magnetic fusion devices.

Numerical Considerations for Neutral Beam Spectroscopy Based Diagnostics in the Stellarator TJ-II

Two neutral-beam based diagnostic techniques are numerically evaluated for the stellarator TJ-II. The evaluation considers the beam from a dedicated diagnostic neutral beam injector that provides a 5 ms pulse of hydrogen accelerated to 30 keV as well as the light throughput of a dual purpose optical system that views the beam as it traverses the microwave, or neutral beam, heated plasmas of this magnetic confinement device. In the first instance, in which the implementation of the beam emission spectroscopy technique is considered for characterizing medium to long wavelength plasma turbulence, detailed estimates are made to determine the Balmer Hα light intensity reaching avalanche photodiode modules. In the second instance, similar detailed estimates are made for the spectrograph-based experimental set-up used for the Motional Stark Emission technique that has been successfully employed to determine the magnitude and direction of the internal magnetic field in TJ-II plasmas. In addition, optical measurements made to evaluate beam energy and composition are described. Finally, estimates and measurements are compared and discussed in order to complete this work.

Measurements of 2D poloidal plasma profiles and fluctuations in ECRH plasmas using the heavy ion beam probe system in the TJ-II stellarator

2D poloidal contour plots of plasma potential, plasma density, and their fluctuations have been measured in low density plasmas sustained by Electron Cyclotron Resonance Heating using a heavy ion beam probe (HIBP) system in the TJ-II stellarator. A HIBP has been used in the new energy scanning mode to obtain the measurements for a 2D poloidal cross section of the stellarator. The 2D map for the absolute plasma potential shows a local maximum in the plasma core as expected in low density plasma scenarios. Fluctuations in the HIBP secondary ion current, as a proxy of plasma density fluctuations, appear both in positive and negative density gradient regions, with a normalized level of density fluctuations higher in the negative density gradient region. The TJ-II innovative experimental setup developed using a dual HIBP diagnostic paves the way for model validation on core plasma potential asymmetries and particle transport and fluctuations under positive and negative density gradient scenarios.

The localization of low order rational surfaces based on the intermittence parameter in the TJ-II stellarator

This work intends to show that the multifractal concept of intermittence can be fruitfully applied to the identification of the radial location of low order rational surfaces inside a magnetically confined plasma. To do so, we make use of Langmuir probe data from a set of experiments in which the rotational transform was scanned dynamically in the TJ-II stellarator. It is shown that up to five rational surfaces can be identified from the data, which is a first in plasma physics, to the best of our knowledge. The effect of the radial electric field on intermittence was also studied using a specific subset of experiments in which the electron density was raised on a shot by shot basis. The observations are contrasted with results from numerical calculations using a resistive magneto-hydrodynamic model to facilitate interpretation.

The impact of edge radial electric fields on edge–scrape-off layer coupling in the TJ-II stellarator

In this letter, we investigate the impact of edge radial electric fields (Er) on turbulence propagation and the coupling between the plasma edge and the scrape-off layer (SOL) during slow electron–ion–electron root transitions in the TJ-II stellarator. The plasma edge was monitored with a set of electrostatic probes and Doppler reflectometry. It is shown, for the first time, that Er does not only affect the radial turbulence correlation length—a well-known effect—but it is also capable of reducing and nearly blocking the propagation of turbulence from the edge into the SOL. Specifically, in the ion root phase (Er < 0 in the edge), the SOL turbulence penetration depth was strongly reduced, as compared to the electron root phase (Er > 0). This result was obtained using a technique based on the transfer entropy, which quantifies the propagation of information. These observations are considered highly relevant in the framework of the understanding of the mechanisms that determine the SOL width, a vital parameter for future fusion reactors.